What is a negative and positive inotropic effect? These are efferent pathways that go to the heart from the centers of the brain and together with them are the third level of regulation.
Discovery story
The influence that the vagus nerves have on the heart was first discovered by the brothers G. and E. Weber in 1845. They found that as a result of electrical stimulation of these nerves, a decrease in the strength and heart rate occurs, that is, an inotropic and chronotropic effect is observed. At the same time, the excitability of the muscle of the heart decreases (Batmotropic negative effect) and with it the speed with which the excitation moves along the myocardium and the conducting system (dromotropic negative effect).
First showed how irritation of the sympathetic nerve affects the heart, I.F. Zion in 1867, and then studied it in more detail by I.P. Pavlov in 1887. The sympathetic nerve affects the same areas of cardiac activity as the vagus, but in the opposite direction. It manifests itself in a stronger contraction of the ventricles of the atria, heart palpitations, increased cardiac excitability and faster excitation (positive inotropic effect, chronotropic, batmotropic and dromotropic effects).
Innervation of the heart
The heart is an organ that is sufficiently innervated. An impressive number of receptors located in the walls of its chambers and in the epicardium give reason to consider it a reflexogenic zone. Two types of mechanoreceptor populations, which are located mainly in the left ventricle and atria, are the most important in the field of sensitive formations of this organ: A receptors that respond to changes in heart wall tension, and B receptors that are excited during its passive extension.
In turn, afferent fibers associated with these receptors are among the vagus nerves. The free sensitive nerve endings located under the endocardium are the terminals of the centripetal fibers that make up the sympathetic nerves. It is generally accepted that these structures are directly involved in the development of a pain syndrome that radiates segmental, which characterizes the attacks of coronary disease. The inotropic effect is of interest to many.
Efferent Innervation
Efferent innervation occurs due to both divisions of the ANS. The sympathetic preanglionic neurons participating in it are located in the gray matter in the three upper thoracic segments in the spinal cord, namely in the lateral horns. In turn, the preanglionic fibers move to the neurons of the sympathetic ganglion (upper chest). The postganglionic fibers together with the parasympathetic vagus nerve create the upper, middle and lower nerves of the heart.
The entire organ is pierced by sympathetic fibers, while they innervate not only the myocardium, but also the components of the conductive system. The parasympathetic preanglionic neurons participating in the cardiac innervation of the body are located in the medulla oblongata. The axons belonging to them move among the vagus nerves. After the vagus nerve enters the chest cavity, branches that are included in the composition of the nerves of the heart depart from it.
Vagus nerve derivatives, which pass along the number of cardiac nerves, are parasympathetic preganglionic fibers. Excitation from them goes to intramural neurons, and then first of all to the components of the conducting system. The effects that are mediated by the right vagus nerve are mainly addressed to the cells of the sinoatrial node, and the left - atrioventricular. Vagus nerves cannot directly affect the ventricles of the heart. The inotropic effect of cardiac glycosides is based on this.
Intramural Neurons
Intramural neurons are also found in large numbers in the heart, and they can be located both singly and assembled in the ganglion. The main number of these cells is located near the sinoatrial and atrioventricular nodes, forming together with the efferent fibers located in the interatrial septum, the intracardiac plexus of nerves. It contains all those elements that are needed in order to close the local reflex arcs. For this reason, the intramural nervous cardiac apparatus is referred in some cases to the metasympathetic system. What is more interesting inotropic effect?
Features of the influence of nerves
At the time when autonomic nerves innervate the tissue of pacemakers, they can affect their excitability and thus cause changes in the frequency of generation of action potentials and heart contractions (chronotropic effect). Also, the influence of nerves can change the speed of electrotonic transmission of excitation, and hence the duration of the phases of the heart cycle (dromotropic effects).
Since the action of mediators in the autonomic nervous system contains a change in energy metabolism and the level of cyclic nucleotides, in general, autonomic nerves can affect the strength of heart contractions, that is, an inotropic effect. Under the influence of neurotransmitters in laboratory conditions, the effect of changing the threshold value of excitation of cardiomyocytes, which is designated as batmotropic, was achieved.
All these ways in which the nervous system affects the contractile activity of the myocardium and cardiac pumping function, of course, are of exceptional importance, but are secondary to the myogenic mechanisms that modulate the effects. Where is the negative inotropic effect?
Vagus nerve and its influence
As a result of stimulation of the vagus nerve, a chronotropic negative effect appears, and against its background, a negative inotropic effect appears (we will consider the preparations below) and dromotropic. There are constant tonic influences of bulbar nuclei on the heart: provided it is cut bilaterally, the heart rate increases from one and a half to two and a half times. If the irritation is strong and prolonged, then the influence of the vagus nerves weakens or ceases altogether. This is called the "slip effect" of the heart from under the corresponding influence.
Plectrum selection
In case of vagus nerve irritation, a chronotropic negative effect is associated with suppression (or deceleration) of pulse generation in the heart rhythm driver of the sinus node. At the ends of the vagus nerve, when it is irritated, a mediator, acetylcholine, is released. Its interaction with muscarinic-sensitive cardiac receptors increases the permeability of the surface of the cell membrane of pacemakers to potassium ions. As a result, hyperpolarization of the membrane appears, which slows down or suppresses the development of slow spontaneous diastolic depolarization, as a result of which the membrane potential reaches a critical level later, which affects the reduction in heart rate. With severe irritations of the vagus nerve, diastolic depolarization is suppressed, hyperpolarization of pacemakers appears, and the heart stops completely.
During vagal influences, the amplitude and duration of the action potential of atrial cardiomyocytes decreases. When the vagus nerve is excited, the atrial irritation threshold rises, automatism is suppressed, and the conduction of the atrioventricular node slows down.
Electrical fiber stimulation
Electrical stimulation of the fibers that extend from the stellate ganglion results in accelerated heart rate and increased myocardial contractions. In addition, the inotropic effect (positive) is associated with an increase in the permeability of the membrane of cardiomyocytes for calcium ions. If the incoming calcium current increases, the level of electromechanical conjugation expands, resulting in an increase in myocardial contractility.
Inotropic drugs
Inotropic drugs are drugs that increase myocardial contractility. The best known are cardiac glycosides (Digoxin). In addition, there are non-glycoside inotropic drugs. They are used only in acute heart failure or when severe decompensation is present in patients with chronic heart failure. The main non-glycoside inotropic drugs are: “Dobutamine”, “Dopamine”, “Norepinephrine”, “Adrenaline”. So, the inotropic effect in the activity of the heart is a change in the force with which it is reduced.